Field of the Invention
The field of the invention is that of the synthesis of carbenes and of their applications, in particular as catalysts or ligands, in:                the preparation of PolyOrganoSiloxanes (POS) by polymerization by ring opening and/or by redistribution and/or polycondensation of POS,        the hydrosilylation of at least one PolyOrganoSiloxane -A- (POS) carrying ethylenic and/or acetylenic unsaturation(s), using at least one polyorganohydrosiloxane -B-, or of other ethylenically and/or acetylenically unsaturated compounds,        hydrogenation,        olefin metathesis,        C—C and/or C—N cross coupling,        asymmetric or enantioselective synthesis,        or organic synthesis: esterification reactions, transesterification reactions, condensation reactions of aldol type, for example.        
The present invention relates to a process for the preparation of carbene by deprotonation of a precursor salt using a strong base.
Throughout the present account, any singular denotes without distinction a singular or a plural.
Description of Related Art
Platinum/carbene complexes are known as catalysts for the hydrosilylation of PolyOrganoSiloxane (POS) comprising ≡Si-vinyl units by means of POS comprising ≡Si—H units. Mention may be made, by way of example, of application PCT WO-A-02/098971, which describes a silicone composition which can be crosslinked to give an elastomer by hydrosilylation in the presence of platinum catalysts formed by a complex C3 or C4:

The carbenes of use as ligands for catalytic metals are made use of in other fields than that of silicones. Thus, patent EP-B-0 971 941 describes catalysts based on Ruthenium and Osmium/carbene complexes for the thermal metathesis of cycloolefins.
In addition, a paper by J. L. Hedrick et al. which appeared in 2002 (JACS, 124, No. 6, pp. 914-915, 2002) teaches that N-heterocyclic carbenes can be used as catalysts for the polymerization of cyclic esters. More specifically, 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene was tested as catalyst for the polymerization of L-lactide, ε-caprolactone and β-butyrolactone in the presence of an alcohol used as initiator.
A more recent and fuller paper (cf. JACS, 125, No. 10, pp. 3046-3056, 2003) describes the preparation of carbene in situ in order to avoid the problems of hydrolysis. The promoters used are of thiazolium, imidazolium and imidazolinium type, respectively giving thiazolecarbene, imidazole-2-ylidenecarbene and imidazolin-2-ylidenecarbene catalysts, when they are brought into the presence of potassium tert-butoxide. Patent application FR-2864543-A discloses carbenes obtained by deprotonation of an imidazolium salt in the presence of a strong base and used for the synthesis of polyorganosiloxane silicones, by polymerization by ring opening and/or redistribution of linear or cyclic polyorganosiloxanes.
Patent application FR-2900153-A discloses a carbene, dicyclohexyldiaminocarbene, obtained by deprotonation of the corresponding imidazolium triflate:
according to the following procedure:

This carbene catalyzes the polycondensation of silanols of the silicone oil type of formula [(CH3)2(OH)SiO1/2]—[(CH3)2SiO2/2]10—[(CH3)2(OH)SiO1/2].
These carbenes of NHC type, obtained by deprotonation of an imidazolium salt using a strong base in a solvent of THF type, are subsequently isolated by filtration, in order to remove the salt formed, followed by recrystallization, indeed even sublimation.
In this respect, patent EP-1 268 440-B1 relates to a process for the preparation of N-heterocyclic carbenes which comprises the heating of a dialkylimidazolium halide with a strong base under reduced pressure. The carbene is distilled during its formation with a horizontal distillation device, optionally distilled with the same device a second time. It can be isolated pure under inert gas or dissolved in an inert solvent (preferably an aromatic solvent).
This process cannot be operated industrially as the horizontal distillation, which uses a laboratory device, is only possible from a few grams to a few tens of grams. Furthermore, the carbene, isolated pure or in inert solvent, is highly sensitive to oxygen and to the slightest trace of moisture and for this reason has a limited lifetime.
These carbenes are highly reactive entities which decompose in the open air, which implies storing them and handling them under an inert atmosphere (argon or nitrogen). Their synthesis is itself also very obviously carried out under an inert atmosphere and the solvent used is dried and freshly distilled beforehand.
Furthermore, the paper by M. A. Schmidt et al., Tetrahedron Letters 49 (2008), 4316-4318, discloses the interaction of N,N′-bismesitylimidazolium-2-yl and alcohols in the presence of benzene:

From this observation, the authors deduce that the stabilizing effect of these interactions can be used for the storage of nitrogenous heterocyclic carbenes, since the removal of the volatiles releases these nitrogenous heterocyclic carbenes. In the case where the alcohol is methanol, this evaporation of the volatiles has to be accompanied by a desolvation in order to restore the carbene-alcohol complex.
In view of this state of the art, the invention is targeted at satisfying at least one of the following essential objectives:                to improve the synthesis of carbenes, that is to say simplifying it, rendering it more economical and resulting in a pure stable solid or liquid form which constitutes a catalytic system which is easy to store and to use and which is also more effective, more profitable and more selective than the carbene catalysts of the prior art;        to obtain carbenes in solution which are stable in the open air for at least several months;        to provide novel metal complexes of carbene which are stable, economical and highly effective catalytically;        to provide processes and catalysts which are stable, economical and highly effective for:                    a. preparation of PolyOrganoSiloxanes (POS) by polymerization by ring opening and/or by redistribution/polycondensation of POS            b. hydrosilylation of at least one PolyOrganoSiloxane -A- (POS) carrying ethylenic and/or acetylenic unsaturation(s), using at least one polyorganohydrosiloxane -B-, or of other ethylenically and/or acetylenically unsaturated compounds            c. hydrogenation            d. olefin metathesis            e. C—C and/or C—N cross coupling            f. enantioselective synthesis            g. organic synthesis                        to provide processes a to g having better conversion yields;        to provide a carbene-based catalytic system which is highly effective, which exhibits at least one of the following characteristics:                    soluble in silicone oils and in particular silicone gums;            simple and relatively inexpensive to synthesize;            stable;            having good resistance to hydrolysis; and which makes it possible:            to polymerize silicones under mild conditions (low temperatures≦100° C.);            to reduce the reaction times, in particular for the preparation of viscous oils and of gums;            to reduce, indeed even to eliminate, residues of catalyst and of its derivatives in the final polymer, in order to prepare silicone polymers of high viscosity and of improved thermal resistance, this being achieved in a profitable manner;            to functionalize a whole POS palette;            to improve the polydispersity of the polymers formed and to favor the formation of linear structures in comparison with cyclic oligomers;            to easily remove possible catalyst residues;            to favor the formation of linear silicone polymers in comparison with the formation of cyclic polymers;            to guarantee high reproducibility;            and to limit the sensitivity to the variability in the starting materials.                        